1. Field of the Invention
The present invention relates to an illumination optical system which illuminates an illumination objective surface with a light flux or light fluxes from, for example, an optical integrator, an exposure apparatus provided with the illumination optical system, an exposure method, and a method for producing a device using the exposure apparatus.
2. Description of the Related Art
For example, when a semiconductor device or the like is produced, in order that a pattern formed on a reticle (or a photomask or the like) is transferred onto a wafer (or a glass plate or the like) coated with a resist to perform the exposure, a projection exposure apparatus of the scanning type (scanning type exposure apparatus) such as a scanning stepper or the like is used, which exposes the wafer while synchronously moving the reticle and the wafer with respect to a projection optical system. Recently, in order to enhance the resolution by shortening the wavelength of the exposure light (exposure light beam), those used as the exposure light source include laser light sources such as the KrF excimer laser (wavelength: 248 nm), the ArF excimer laser (wavelength: 193 nm) and the like. Further, an exposure apparatus has been also developed, which employs, as the exposure light, the extreme ultraviolet light (hereinafter referred to as “EUV light (EUV light beam)”) having a wavelength of, for example, not more than about 100 nm. Any one of the laser light sources and the light source of the EUV light beam is the pulsed light source.
When the pulsed light beam is used as the exposure light in the scanning type exposure apparatus, any unevenness in the totalized amount of exposure (unevenness in the scanning intensity) arises at respective points on the wafer after the scanning exposure resulting from, for example, the unevenness in the intensity of each of the pulsed lights or the jitter (temporal fluctuation) of the scanning velocity of the stage. A technique for reducing the unevenness in the scanning intensity is the illumination of an illumination area on the reticle with the light flux from the illumination optical system while providing an intensity distribution which has a trapezoidal shape in the scanning direction. For this purpose, those hitherto known include a technique in which a blind is arranged at a position slightly separated from a surface of the pattern (pattern surface) of the reticle or the conjugate plane of the pattern surface, a technique in which illumination areas formed by a plurality of elements included in the optical integrator are deviated or shifted in the scanning direction (see, for example, U.S. Pat. No. 7,006,595), a technique in which the light-exit directions of the light fluxes of the plurality of elements are shifted in the scanning direction (see, for example, Japanese Patent Application Laid-open No. 10-92730), and a technique in which the focal lengths or the sizes of apertures of the plurality of elements are made to differ (see, for example, Japanese Patent Application Laid-open No. 10-189431).
In general, in order to illuminate the respective points on the illumination area by the illumination optical system under a same illumination condition, i.e., with the light fluxes having same distribution of the aperture angle or opening angle (numerical aperture), it is necessary that the light fluxes, from the entire area included in the aperture of the aperture diaphragm of the illumination optical system (hereinafter referred to as “illumination σ”), are irradiated onto the respective points respectively. However, in any one of the conventional techniques for obtaining the trapezoidal shape of the intensity distribution of the illumination area in the scanning direction, the lack or deficiency of the illumination σ arises such that only the light fluxes, which come from a part of the area (partial areas) included in the illumination σ, are radiated onto an area in which the intensity distribution is inclined, thus arising a problem such that the imaging characteristic of the image of such area is deteriorated.
The influence of the deterioration of the imaging characteristic as described above is mitigated owing to the averaging effect brought about by the scanning exposure. However, when the ratio of the area having the inclined intensity distribution in the illumination area is increased in order to further suppress the unevenness in the scanning exposure, for example, especially in the EUV exposure apparatus, it is feared that any necessary imaging characteristic might not be obtained.
Taking the foregoing circumstances into consideration, the present invention provides several aspects, an object of which is to provide an illumination optical system, an exposure apparatus, and an exposure method in which the intensity distribution of the illumination area can be defined or set to be a nonuniform distribution, and in which respective points included in the illumination area can be illuminated under an approximately same illumination condition (with light fluxes having a same aperture angle or opening angle distribution), as well as a technique for producing a device using the exposure apparatus.